Gas sensors are employed in a variety of applications requiring qualitative and quantitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensors are employed within the internal combustion control system of the automobile to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
The conventional electrochemical type of oxygen sensor employed in automotive applications utilizes a thimble-shaped electrochemical galvanic cell to determine the relative amounts of oxygen present in the exhaust stream, as disclosed in U.S. Pat. No. 3,844,920 to Burgett et al. This type of oxygen sensor comprises an ionically conductive solid electrolyte material, typically zirconia stabilized by the addition of yttria, a porous electrode coating on the exterior contacting the exhaust or measuring gas, and a porous electrode coating on the interior contacting a known concentration of reference gas. The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes.
Currently, these thimble-shaped, electrochemical-type oxygen sensors are employed in the exhaust gas system of an internal combustion engine to determine qualitatively whether the engine is operating at either of two conditions: (1) a fuel rich or (2) a fuel lean condition, as compared to stoichiometry. After equilibration, the exhaust gases from these two operating conditions have two widely different oxygen partial pressures. This information is provided to an air-to-fuel ratio control system, so that it can provide an average stoichiometric air-to-fuel ratio between the two conditions. However, due to the increasing demands for improved fuel utilization and emissions control, it is desirable to operate internal combustion engines exclusively within lean combustion conditions, i.e., air-to-fuel ratios between 15:1 and 25:1, where changes in the after-combustion oxygen partial pressures are only slight and gradual. The current oxygen sensor is not sensitive enough for this operating environment, since it merely provides the internal combustion control system with an output signal corresponding to the gross determination of either a rich or lean air-to-fuel ratio.
To be an effective component of the internal combustion control system operating exclusively within lean combustion conditions, the oxygen sensor must be extremely sensitive and capable of rapid, precise, absolute oxygen concentration measurements. It is desirable that the response time of the sensor be less than 0.1 second at a minimum temperature of 300.degree. C. and a maximum oxygen concentration at the sensing electrode of about eight percent. The sensor must also be structurally durable to withstand the harsh automotive environment.
Internal reference oxygen sensors have been devised for lean engine operation and typically comprise two solid electrolyte galvanic cells; the first galvanic cell senses the gas to be measured, commonly referred to as the sense cell, while the second galvanic cell generates an accurately known internal gas reference, commonly referred to as the pump cell. The accurately known internal gas reference is generated by electrochemically pumping oxygen gas into and out of a hermetically sealed, fixed volume chamber by means of the pump cell. An external power source provides a potential across the solid electrolyte body of the pump cell. Electrons supplied at one electrode ionize gas molecules at the interface between that negatively biased electrode and the solid electrolyte. The gas ions are then transported through the solid electrolyte by ionic conduction. At the other electrode, the gas ions lose electrons and recombine into gas molecules. By reversing the polarity of the external circuit, oxygen gas can be transported in the other direction and subsequently pumped out of the fixed volume chamber. The partial pressure, i.e., concentration, of the oxygen gas in the gas mixture can be measured by simultaneoulsy sensing the oxygen partial pressure differential between the internal reference gas chamber and the gas mixture to be measured with the sense cell.
Internal reference, solid electrolyte oxygen sensors may be operated in various modes to determine gas concentration measurements. One method is to pump oxygen into the internal reference gas chamber with the pump cell until the voltage output at the sense cell equals some threshold value. The period of time required to pump that amount of oxygen into the chamber is related to the oxygen partial pressure in the exhaust gas. An alternative method is to maintain a constant oxygen pressure in the internal reference gas chamber and determine exhaust oxygen concentration from the voltage output measurements at the sense cell.
If one elects to cycle oxygen out of and back into the reference gas chamber each time one chooses to measure oxygen partial pressure in a gas mixture, sensor response time will be proportional to the volume of the internal reference gas chamber, i.e., the number of gas molecules pumped into and out of the chamber. Therefore, it is desirable to provide a chamber of minimum volume so that the sensor response time is minimized. A prior improved oxygen sensor disclosed in U.S. Pat. No. 4,724,061 entitled "Automotive, Internal Reference, Solid Electrolyte, Lean Oxygen Sensor" laterally positions both the sense and pump cell components on a single substrate and uses conventional thin film deposition techniques with a thick film capping layer to produce an internal reference gas chamber of minimal volume. Our invention further improves and miniaturizes the internal reference gas chamber by disclosing a method of making an internal reference gas chamber comprised of the pores of a porous material that spaces adjacent electrodes of the pump and sense cells. The volume of the internal reference gas chamber and amount of oxygen pumped into and out of the chamber during one pump cycle of the oxygen sensor are further reduced, maximizing the efficiency of the oxygen sensor.